Temperature detector utilizing optical fibers is one of extremely high necessity as measuring apparatus to replace the conventional electric temperature detector under bad environment where electromagnetic induction noise or electric insulation comes into problem. Or again, in a remote measuring system utilizing a communication system of low-loss optical fiber, an element which transfer external physical amount, such as, temperature directly into light signal without once transforming into electric signal becomes important one.
FIG. 1 shows a configuration of conventionally proposed temperature detector using optical fiber for such use. The temperature detector consists of a birefringent crystal 1 for temperature detection, optical fibers 2, 7, lens 3, 6, sensor part consisting of a polarizer 4 and an analyzer 5 with their transparency polarization directions having right angle each other, light emitting part 2 (not shown) connected to the other end of the optical fiber 2, and received light signal processing part (not shown) connected to the other end of the optical fiber 7.
Operation of the temperature detector of such configuration is elucidated; for instance at a temperature of T.sub.1 the direction of polarization of light passing the birefringent crystal 1 does not substantially change, accordingly the direction of polarization of light after passing the crystal 1 is in the state having right angle to the direction of transmitted polarized light of the analyzer 5, the light passing is hindered by the analyzer 5 and light output intensity of the optical fiber 7 becomes minimum. Next, as temperature rises the direction of polarization of light passing the birefringent crystal 1 becomes to change during the passing; for instance at the temperature T.sub.2 (T.sub.1 &lt;T.sub.2) a part of light which passes the birefringent crystal 1 becomes of the same polarization direction as the transmitted polarized light of the analyzer 5 and light output intensity of the optical fiber becomes maximum. Since the light output of the optical fiber 7 changes responding to temperature of the birefringent crystal 1 in this manner, by detecting the light intensity the temperature of the part where the birefringent crystal 1 is located can be known. Incidentally, in the said configuration, transparency polarizing direction of the polarizer 4 and analyzer 5 are set to have right angle each other, but these directions are set parallelly.
In such temperature detector, as the light source a use of light emitting diode in which characteristic is stable and price is cheap, is preferable, but in the conventional way of using the light emitting diode, there have been made no study as to which material for the birefringent crystal 1 may be used.
Furthermore, there has been made no study as to in which range of characteristic curve of temperature and light output intensity of the birefringent crystal 1 an operation range should be determined in order that a highest accuracy measurement can be made.
Still furthermore, there has been made no study at all as to how measurement error due to fluctuation of loss of light passing through optical fibers 2, 7 is prevented.
Summarizing the above, although the temperature detector using the optical fiber is observed hopeful as measurement apparatus usable under a bad condition where electromagnetic induction noise, electric insulation, etc., are the question, it has not come to actual use since studies of the above-mentioned points have not been made so far.